Heavily armed troops displaying no identifying insignia and local pro-Russian militants stand guard outside a local government building on March 2 in Simferopol, Ukraine. / Sean Gallup, Getty

by Olga Rudenko, Jacob Resneck and Jabeen Bhatti, Special for USA TODAY

by Olga Rudenko, Jacob Resneck and Jabeen Bhatti, Special for USA TODAY

SEVASTOPOL, Ukraine - U.S. Secretary of State John Kerry and European foreign ministers announced plans Sunday for talks in Kiev aimed at easing tensions as Ukraine put its military on high alert and appealed for international help against a feared full-blown invasion by Russia.

Kerry issued a tweet Sunday saying he would depart Monday night for discussions Tuesday. German chancellor Angela Merkel informed Russian President Vladimir Putin of the decision to send German, French and Polish diplomats to talks in the violence-torn country.

Kerry called on Putin to pull back from "an incredible act of aggression."

"It's the time for diplomacy,'' Kerry said on Twitter. "Nobody wants this to spiral in a worse direction.''

However, Russia showed no signs of withdrawing the thousands of troops it has deployed to the Crimea region of Ukraine. A senior White House official - who spoke on the condition of anonymity to discuss the sensitive administration efforts - said Russia has moved 6,000 troops into the region.

Ukrainian military reservists were ordered to active duty while in the Crimean region in the south of the country, road traffic was blocked and telecommunications remained sporadic - two days after communication centers were seized by armed men.

Earlier Sunday, hundreds of armed men in trucks and armored vehicles surrounded a Ukrainian military base in Crimea, blocking its soldiers from leaving in a tense standoff when Ukrainians placed a tank at the base's gate.

"The situation is very serious - the Russian army is blocking military bases of Ukraine in Crimea. â?¦ They issued an ultimatum demanding that our soldiers disarm themselves or the bases will be stormed," said Ukraine's interim president and parliament speaker, Oleksandr Turchinov, on Sunday.

Speaking after a closed session of parliament in Kiev, Ukrainian Prime Minister Arseniy Yatsenyuk said his country is "on the brink of disaster." He said that "if (Russian) President Putin wants to be the president who starts a war between two friendly and neighboring countries, he has reached his target within a few inches."

"This is not a threat, this is actually a declaration of war to my country," he added.

On Sunday, the interim government in Kiev said the military must be combat-ready as Russian soldiers reportedly surrounded Ukrainian military and naval bases in Crimea, the strategic Black Sea peninsula in Ukraine.

"We need a unified army â?¦ we believe that discipline and coordination is now extremely important," said Andriy Paruby, secretary of Ukraine's security council, on Sunday.

The new head of the Ukrainian Navy was fired Sunday after he swore allegiance to the unrecognized leader of Crimea and ordered his troops in Crimea to lay down their arms, according to government officials in Kiev.

Denys Berezovsky, who was appointed last week, is under investigation for treason, and has been replaced by Rear Admiral Sergiy Haiduk, officials from the Security and Defense Council said.

A government body in the farthest eastern region of Luhansk announced it considers the interim government in Kiev "illegitimate," the latest district to challenge the new leadership in the Western-oriented capital.

Meanwhile, world leaders reacted to the conflict as NATO held an emergency meeting, and France and Britain announced they had pulled out of preparatory talks for the upcoming Group of Eight summit in Sochi, Russia.

On Sunday, Kerry said the Obama administration is working with U.S. allies to hit Russia with economic penalties if it continues its aggression in the Crimean region.

American allies "are prepared to go to the hilt in order to isolate Russia with respect to this invasion," Kerry said on CBS' Face The Nation. "They're prepared to put sanctions in place, they're prepared to isolate Russia economically."

On Saturday, the U.S. suspended its participation in the G-8 talks as it called on Moscow to withdraw its forces from the region and "refrain from any interference elsewhere in Ukraine."

Speaking by phone with Putin for the first time since this crisis escalated, President Obama expressed concern over "Russia's clear violation of Ukrainian sovereignty and territorial integrity," according to a White House statement.

However, Putin remained defiant, telling Obama that not only can Russia send its troops to Crimea, but to all of predominantly Russian-speaking eastern Ukraine due to "the existence of real threats" to Russian citizens in Ukrainian territory, according to a statement on the Kremlin's website.

On Sunday, government buildings, airports and communications centers in Crimea continued to be held by groups of armed men. Local journalists reported that attempts to access the Crimea were unsuccessful after they were stopped and turned back at checkpoints.

Telecom operator Ukrtelecom said their lines had been damaged and that their technicians were "doing everything in their power to normalize the network operator and ensure a speedy recovery of the lines on the peninsula."

NATO Secretary General Anders Fogh Rasmussen said in a statement Sunday that Russia's action in Crimea "violates the principles of the United Nations Charter. It threatens peace and security in Europe. Russia must stop its military activities and its threats."

Late Saturday, small rallies were held in Crimean cities expressing support for Russian help. Stories of hard-line Ukrainian nationalists from Kiev's Independence Square planning attacks on ethnic Russians have been circulating for weeks, and many people in this port city have either welcomed the Russian intervention or said they have no fear of Moscow's forces.

"This isn't really a crisis because all of Sevastopol was afraid of the Maidan movement," said waiter Konstantin Solovev, 28. "We heard many stories that those people were threatening this city."

Even so, some locals created a petition signed by tens of thousands telling Russia they didn't need Russian help.

"Dear Mr. President of the Russian Federation, Vladimir Putin, We ethnic Russians and Russian-speaking Ukrainian nationals do not need other countries to defend our interests," the petition read. "We are grateful to you for support however would like to inform you that nobody has ever infringed our rights on Ukrainian territory ... therefore, we would ask you to not raise an internal question for our country which is not a burning issue for us ... Not to mention bringing troops in to regulate a conflict which you may see but which is not visible to us."